In recent years, a liquid crystal display including a so-called environmental sensor (in the following, referred to as an “ambient sensor”) for detecting the intensity of light in the surroundings (in the following, referred to as “ambient light”) has been known (for example, see Patent documents 1 and 2). In such a liquid crystal display, the intensity of light radiated from a backlight system is adjusted according to the intensity of the detected ambient light.
More specifically, in the case of a transparent liquid crystal display, the light intensity of the backlight system is raised in a light environment such as the outdoors and reduced in a relatively dark environment such as the nighttime and indoors. Thus, a liquid crystal display provided with the ambient sensor achieves improved screen visibility lower power consumption and longer lifetime. The liquid crystal display provided with the ambient sensor is useful particularly as a display device of portable terminals (mobile phones, PDAs, hand-held game machine, etch) that are often taken outdoors for use.
Examples of the ambient sensor include optical sensors such as a photodiode and a phototransistor. The optical sensor can be mounted on the liquid crystal display by placing an optical sensor provided as a discrete component on a display panel (for example, see Patent document 3). Also, in recent years, in order to cut the manufacturing cost and miniaturize the display device by reducing the number of components, an attempt has been made to form an optical sensor monolithically on an active matrix substrate constituting a display panel (for example, see Patent document 4). In this case, the optical sensor is formed by utilizing the process of forming an active element (TFT).
Herein, the configuration of a liquid crystal display on which an optical sensor is mounted will be described with reference to FIG. 14. FIG. 14 shows the configuration of a conventional liquid crystal display on which an optical sensor is mounted. In FIG. 14, the configuration of a liquid crystal display panel constituting the liquid crystal display is illustrated schematically.
As shown in FIG. 14, the liquid crystal display panel is constituted by sandwiching a liquid crystal layer 102 between an active matrix substrate 101 and a counter substrate 103. A region of the active matrix substrate 101 that is in contact with the liquid crystal layer 102 serves as a display region. In the display region, a plurality of pixels are arranged in a matrix. Each of the pixels includes an active element and a pixel electrode. Further, an optical sensor 104 is formed monolithically in a region surrounding the display region (in the following, referred to as a “peripheral region”) by utilizing the process of forming the active elements.
A backlight system is disposed on the back side (the side on which no active element is formed) of the active matrix substrate 101. In the example of FIG. 14, the backlight system is of a sidelight type and mainly includes a light guide plate 108 and a light source 105. The light source 105 is constituted by a fluorescent lamp 106 and a lamp reflector 107. Further, a reflective sheet 109 is attached to the lower surface and sides (not shown) of the light guide plate 108. Moreover, a diffusing sheet 110 and a prism sheet 111 are attached to the upper surface (an emission surface) of the light guide plate 108 in this order
Light emitted from the light source 105 is reflected inside the light guide plate 108 and emitted from the upper surface (the emission surface) of the light guide plate 108. The light emitted from the emission surface of the light guide plate 108 first enters the diffusing sheet 110 and is diffused. This reduces brightness unevenness. Further, the light that has passed through the diffusing sheet 110 is refracted by a prism sheet 111 so as to be turned into light that is substantially parallel with the normal to the emission surface, and passes through the active matrix substrate 101, the liquid crystal layer 102 and the counter substrate 103 in this order.
Also, at this time, a control device (not shown in FIG. 14) of the backlight system adjusts the intensity of light emitted from the light source 105 in the backlight system according to the intensity of ambient light detected by the optical sensor 104. Therefore, using the liquid crystal display as shown in FIG. 14, it is possible to improve the screen visibility and achieve lower power consumption and longer lifetime.    Patent document 1: JP 4(1992)-174819 A    Patent document 2: JP 5(1993)-241512 A    Patent document 3: JP 2002-62856 A (FIGS. 12 to 14)    Patent document 4: JP 2002-175026 A (FIG. 12)